To perform position location in wireless cellular networks (e.g., a cellular telephone network), several approaches perform trilateration based upon the use of timing information sent between each of several base stations and a mobile device, such as a cellular telephone. One approach, called Advanced Forward Link Trilateration (AFLT) or Enhanced Observed Time Difference (EOTD), measures at the mobile device the times of arrival of signals transmitted from each of several base stations. These times are transmitted to a Position Determination Entity (PDE) (e.g., a location server), which computes the position of the mobile device using these times of reception. The transmit times at these base stations are coordinated such that at a particular instance of time, the times-of-day associated with multiple base stations are within a specified error bound. The accurate positions of the base stations and the times of reception are used to determining the position of the mobile device.
FIG. 1 shows an example of an AFLT system where the times of reception (TR1, TR2, and TR3) of signals from cellular base stations 101, 103, and 105 are measured at the mobile cellular telephone 111. This timing data may then be used to compute the position of the mobile device. Such computation may be done at the mobile device itself, or at a location server if the timing information so obtained by the mobile device is transmitted to the location server via a communication link. Typically, the times of receptions are communicated to a location server 115 through one of the cellular base stations (e.g., base station 101, or 103, or 105). The location server 115 is coupled to receive data from the base stations through wireless network 113 (e.g., a mobile switching center), circuit switched network 117 (e.g., land line Public Switched Telephone Network) and/or packet switched network 119 (e.g., packet data service node). The location server may include a base station almanac (BSA) server, which provides the location of the base stations and/or the coverage area of base stations. Alternatively, the location server and the BSA server may be separate from each other; and, the location server communicates with the base station to obtain the base station almanac for position determination. A mobile switching center may provide signals (e.g., voice communications) to and from a land line Public Switched Telephone Network (PSTN) or a packet data service node so that signals may be conveyed to and from the mobile telephone to other telephones (e.g., land line phones on the PSTS or other mobile telephones). The location server may also monitor emissions from several of the base stations in an effort to determine the relative timing of these emissions.
In another approach, called Time Difference of Arrival (TDOA), the times of reception of a signal from a mobile device is measured at several base stations (e.g., measurements taken at base stations 101, 103 and 105). FIG. 1 applies to this case if the arrows of TR1, TR2, and TR3 are reversed. This timing data may then be communicated to the location server to compute the position of the mobile device.
Yet a third method of doing position location involves the use in the mobile device of a receiver for the United States Global Positioning Satellite (GPS) system or other Satellite Positioning System (SPS), such as the Russian GLONASS system and the proposed European Galileo System, or a combination of satellites and pseudolites. Pseudolites are ground-based transmitters, which broadcast a PN code (similar to a GPS signal) modulated on an L-band carrier signal, generally synchronized with SPS time. Each transmitter may be assigned a unique PN code so as to permit identification by a remote receiver. Pseudolites are useful in situations where SPS signals from an orbiting satellite might be unavailable, such as tunnels, mines, buildings or other enclosed areas. The term “satellite”, as used herein, is intended to include pseudolite or equivalents of pseudolites, and the term GPS signals, as used herein, is intended to include GPS-like signals from pseudolites or equivalents of pseudolites. Such a method using a receiver for SPS signals may be completely autonomous or may utilize the cellular network to provide assistance data or to share in the position calculation. As shorthand, we call these various methods “SPS”. Examples of such methods are described in U.S. Pat. Nos. 6,208,290; 5,841,396; 5,874,914; 5,945,944; and 5,812,087. For instance, U.S. Pat. No. 5,945,944 describes a method to obtain from cellular phone transmission signals accurate time information, which is used in combination with SPS signals to determine the position of the receiver; U.S. Pat. No. 5,874,914 describes a method to transmit the Doppler frequency shifts of in view satellites to the receiver through a communication link to determine the position of the receiver, U.S. Pat. No. 5,874,914 describes a method to transmit satellite almanac data (or ephemeris data) to a receiver through a communication link to help the receiver to determine its position; U.S. Pat. No. 5,874,914 also describes a method to lock to a precision carrier frequency signal of a cellular telephone system to provide a reference signal at the receiver for SPS signal acquisition; U.S. Pat. No. 6,208,290 describes a method to use an approximate location of a receiver to determine an approximate Doppler for reducing SPS signal processing time; and, U.S. Pat. No. 5,812,087 describes a method to compare different records of a satellite data message received at different entities to determine a time at which one of the records is received at a receiver in order to determine the position of the receiver. In practical low-cost implementations, both the mobile cellular communications receiver and the SPS receiver are integrated into the same enclosure and, may in fact share common electronic circuitry.
In yet another variation of the above methods, the round trip delay (RTD) is found for signals that are sent from the base station to the mobile device and then are returned. In a similar, but alternative, method the round trip delay is found for signals that are sent from the mobile device to the base station and then returned. Each of these round-trip delays is divided by two to determine an estimate of the one-way time delay. Knowledge of the location of the base station, plus a one-way delay constrains the location of the mobile device to a circle on the earth. Two such measurements from distinct base stations then result in the intersection of two circles, which in turn constrains the location to two points on the earth. A third measurement (even an angle of arrival or cell sector) resolves the ambiguity.
A combination of either the AFLT or TDOA with an SPS system is called a “hybrid” system. For example, U.S. Pat. No. 5,999,124 describes a hybrid system, in which the position of a cell based transceiver is determined from a combination of at least: i) a time measurement that represents a time of travel of a message in the cell based communication signals between the cell based transceiver and a communication system; and, ii) a time measurement that represents a time of travel of an SPS signal.
Altitude aiding has been used in various methods for determining the position of a mobile device. Altitude aiding is typically based on a pseudo-measurement of the altitude. The knowledge of the altitude of a location of a mobile device constrains the possible positions of the mobile device to a surface of a sphere (or an ellipsoid) with its center located at the center of the earth. This knowledge may be used to reduce the number of independent measurements required to determine the position of the mobile device. For example, U.S. Pat. No. 6,061,018 describes a method where an estimated altitude is determined from the information of a cell object, which may be a cell site that has a cell site transmitter in communication with the mobile device.